Hard disk drives are used in almost all computer system operations, and recently even in consumer electronic devices such as digital cameras, video recorders, and audio (MP3) players. In fact, most computing systems are not operational without some type of hard disk drive to store the most basic computing information such as the boot operation, the operating system, the applications, and the like. In general, the hard disk drive is a device which may or may not be removable, but without which the computing system will generally not operate. However, some computer systems exist in which the hard drive function is performed by compact flash memory.
The basic hard disk drive model was established approximately 50 years ago. The hard drive model includes a plurality of storage disks or hard disks vertically aligned about a central core that can spin at a wide range of standard rotational speeds depending on the computing application in which the hard disk drive is being used. Commonly, the central core is comprised, in part, of a spindle motor for providing rotation of the hard disks at a defined rotational speed. A plurality of magnetic read/write transducer heads, commonly one read/write transducer head per surface of a disk, where a head reads data from and writes data to a surface of a disk, are mounted on actuator arms.
Data is formatted as written magnetic transitions (information bits) on data tracks evenly spaced at known intervals across the disk. An actuator arm is utilized to reach out over the disk to or from a location on the disk where information is stored. The complete assembly at the extreme of the actuator arm, e.g., the suspension and magnetic read/write transducer head, is known as a head gimbal assembly (HGA).
In operation, pluralities of hard disks are rotated at a set speed via a spindle motor assembly having a central drive hub. Current types of spindle motors include, but are not limited to, various types of bearing systems having a rotating or fixed shaft. Additionally, there are channels or tracks evenly spaced at known intervals across the disks. When a request for a read of a specific portion or track is received, the hard disk drive aligns a head, via the actuator arm, over the specific track location and the head reads the information from the disk. In the same manner, when a request for a write of a specific portion or track is received, the hard disk drive aligns a head, via the actuator arm, over the specific track location and the head writes the information to the disk.
Many of today's hard disk drives, particularly those hard disk drives that are designed to operate at high revolutions, e.g., above 10,000 rpm, include a spindle motor comprising, in part, a fluid dynamic bearing (FDB). An FDB may have a rotating sleeve (fixed shaft) or a rotating shaft (fixed sleeve). It is well known in the art that an FDB provides improved functionality and performance compared with a spindle motor having a ball bearing system.
In particular, it is common for an FDB with a rotating or fixed shaft to be configured with internally disposed grooves that may be configured in, but is not limited to, a herringbone-pattern or a spiral pattern. Grooves (recesses or troughs) and lands (non-recessed areas) are oriented in such an arrangement as to cause pressure between the rotor and stator and it is this pressure that allows the rotor to spin in a stable manner around the shaft, ideally without contact between the stator and the rotor. It is also common for the rotor to be symmetrical from top to bottom. As the rotor spins, upon which the grooves may be located, the grooves cause pressurization of the fluid (oil, air, or other substance) inside the FDB. This allows the rotor to spin freely around the fixed shaft. This is similar to being suspended in oil, with no solid contact between the stator and the rotor.
With reference to a herringbone-patterned FDB, it is common for one of the grooves to be longer than the other grooves, referred to as the unbalanced length or portion. The reason for the unbalanced length is to accommodate the oil air interface (OAI), also referred to as the meniscus. A fixed shaft design (FSD) type of a fluid dynamic bearing (FDB) enables the rotor to spin very smoothly around the shaft. A FSD type FSB is commonly, but not always, implemented in high end server or enterprise type hard disk drives, e.g., those hard disk drives having extremely high capacity and fast rotating speeds. These types of hard disk drives are commonly implemented in server farms and hard disk drive farms. It is not uncommon for a hard disk drive configured with a FSD FDB to reach rotating speeds in excess of 15,000 rpm.
Most fixed shaft design fluid dynamic bearings (FSD FDBs) currently available have groove angles that are constant, relative to bearing shaft perpendicularity. It is well known in the art that the groove angle and bearing stiffness are interrelated. Bearing stiffness within an FDB describes the tendency/ability of the bearing to restore itself, e.g., correct itself relative to a force applied. This is commonly referred to as radial stiffness. Radial stiffness correlates to bearing groove angle. As the groove angle is decreased, bearing stiffness is reduced and when the groove angle is increased, radial stiffness increases. It is also well knows that a groove angle of approximately twenty degrees provides maximum stiffness without detrimentally affecting FDB operation. It is noted that when the groove angle exceeds twenty degrees, the stiffness decreases. It is also well known that a groove angle of less than five degrees will render most FDBs inoperable.
It is well known that when there is an oil and air interface, there is surface tension between the two substances. It is this surface tension that stabilizes the oil in the bearing. When the oil air interface (OAI) is located among grooves, the interface may deform such that it is drawn into the grooves. This is problematic because this is where the danger of air ingestion can occur. With reference to the oil air interface (OAI), the OAI is substantially horizontal when the bearing is not in operation, relative to the vertical axis of the FDB. During FDB operation, the OAI exhibits a wobbly or wave-like shape, similar to a sinusoidal waveform, such that the OAI rises and falls within the bearing system. As the motor is spinning, the oil is drawn into the grooves and pushed outward over the lands. Further, as the speed of the motor increases, the wave-like phenomenon of the OAI becomes extreme, such that the surface of the OAI can form cusps, as shown in prior art FIGS. 8, 9, 10 and 11.
FIG. 7 is an isometric view of the fluid/liquid 10 in a typical FDB, if one could make rigid the fluid and remove all the metal parts there from. FDB fluid 10 shows a spiral pattern 8 that is representative of grooves that would be disposed upon a surface of the thrust bearing of an FDB. FDB liquid 10 also shows a herringbone pattern 9 that is representative of grooves that would be located on a journal or radial surface of the shaft of the FDB. Also shown in FDB fluid 10 is an oil air interface (OAI) 7. OAI 7 is located near the opening of an FDB when the FDB is idle, and the OAI migrates into the herringbone groove pattern during operation of the FDB. It is noted that the groove angles are constant with the exception of the rounding near the apex of herringbone pattern 9.
FIG. 8 shows a line 11 representing an oil air interface in which a bearing system is at rest. FIG. 9 shows a line 12 representing an oil air interface in which the bearing system is now rotating. FIG. 9 also includes a cusp 22 that is formed as the fluid is drawn into a groove within the bearing system as the rotational speed of the rotor increases. FIG. 10 includes a line 13 representing an oil air interface in which the bearing system is rotating faster than the bearing system shown in FIG. 9. FIG. 10 also includes a cusp 23, formed by the fluid being drawn into a groove in which cusp 23 is deeper and sharper than cusp 22 of FIG. 9. The cusp increase is caused by the increased rotational speed of the rotor within the bearing system. It is well knows that increased rotational speeds can cause the OAI to become very sharp, such that it can draw air into the liquid. FIG. 11 illustrates such an occurrence. FIG. 11 includes a line 14 representing an oil air interface in which the bearing system is rotating faster than that shown in FIG. 10. FIG. 11 also includes a cusp 24, formed by the fluid being drawn into a groove, in which the cusp is deeper and sharper that cusp 24 of FIG. 9. FIG. 11 further includes a plurality of bubbles 34 formed as a result of cusp 24 peaking because of increased rotating speed of the bearing system.
When air is drawn into the liquid, this is very detrimental to the operation and function of the FDB. For the liquid, e.g., oil, to operate properly, the liquid needs to be very incompressible, thus providing high stiffness during operation. Incompressibility refers to the characteristic of the liquid to resist change in volume as a result of a pressure applied thereto. When air bubbles are formed in a liquid, the liquid becomes gummy or mushy, similar to hydraulic brakes when air gets into the brake fluid, such that it is quite difficult to apply firm pressure.
During FDB operation, there is a capillary number that is a key parameter in this type of flow phenomenon, e.g., the formation of a cusp. The capillary number is derived from the viscosity of the liquid times speed with which the grooves pass the stator divided by surface tension. The higher the value of the capillary number, the greater the chance of air ingestion occurring during operation.
Accordingly, many of today's currently available FDBs are prone to the phenomenon of air ingestion. Thus, a need exists for an FDB that can substantially reduce instances of air ingestion occurring during operation.